From your dashboard, click Forms > New Form. A box will pop up asking you to give your form a title and a description. Fill those fields out and click Create Form. This will lead you into a robust drag and drop interface where you can choose your form fields from the right-side menu.
Gravity Forms has detailed documentation on each of their available fields.
Warning: If you delete any fields from an existing form you will also delete all the past data collected by that field. Export your entries before you begin.